1. Field of the Invention
The present invention relates to a light emitting panel in which a light emitting element formed on an insulating surface is sealed between a substrate and a cover member. The invention also relates to a light emitting module obtained by mounting a driving circuit to the light emitting panel. xe2x80x98Light emitting devicexe2x80x99 is herein the generic term for the light emitting panel and for the light emitting module. Specifically, the present invention relates to improving efficiency in taking out light from a light emitting element.
2. Description of the Related Art
In recent years, the technique of forming TFTs on a substrate has made a great advance and application of TFTs to active matrix display devices (light emitting devices) is being developed. TFTs formed of polysilicon films, in particular, have higher field effect mobility (also referred to as mobility) than conventional TFTs that use amorphous silicon films and accordingly can operate at high speed. Therefore pixels now can be controlled by a driving circuit formed on the same substrate on which the pixels are formed instead of a driving circuit external to the substrate as in the past.
Thus having various circuits and elements formed on the same substrate, an active matrix light emitting device provides a lot of advantages including reduction of manufacture cost, reduction in size of electro-optical device, raise in yield, and improvement of throughput.
An active matrix light emitting device using a light emitting element as a self-luminous element is particularly actively researched.
In this specification, a light emitting element has an organic compound layer sandwiched between a pair of electrodes (an anode and a cathode). The organic compound layer may take a laminate structure. As an example, a laminate structure consisting of a hole transporting layer, an organic compound layer, and an electron transporting layer can be given. The term organic compound layer in this specification includes layers for carrier injection, layers for carrier transportation, and layers for carrier recombination all. Luminescence obtained from an organic compound layer is classified into light emission upon return to the base state from singlet excitation (fluorescence) and light emission upon return to the base state from triplet excitation (phosphorescence). The present invention is applicable to a light emitting device using fluorescence and a light emitting device using phosphorescence both.
Heat, light, moisture, oxygen, and the like accelerate degradation of an organic compound layer of a light emitting element. For that reason, in general, a light emitting element is formed after a wiring line and a TFT are formed in a pixel portion in manufacturing an active matrix light emitting device.
After the light emitting element is formed, the substrate on which the light emitting element is formed is bonded to a cover member and sealed (packaged) using a seal member or the like so as not to expose the light emitting element to the outside air.
Once the airtightness is enhanced by packaging or other processing, a connector (FPC, TAB, or the like) is attached to connect a terminal led out of the light emitting element or a circuit formed on the substrate to an external signal terminal. The active matrix light emitting device is thus completed.
Now, a description is given on refraction of light with reference to FIG. 15. As shown in FIG. 15, the angle of refraction of light is determined by the angle of incident light (angle of incident) and index of refraction of the medium thereof. This relation follows Mathematical Expression 1 (Snell""s Law) below. When light (incident light) enters, at an angle of xcex81, a medium 801 having an index of refraction of n1 and exits a medium 802 having an index of refraction of n2, the light (refracted light) has an angle of xcex82 satisfying the Expression 1 below.
Mathematical Expression 1
n1*sin xcex81=n2*sin xcex82xe2x80x83xe2x80x83(1)
The angle of incident xcex81 that makes the angle xcex82 of the refracted light or transmitted light 90xc2x0 is called a critical angle. When the angle of incident xcex81 to the medium 802 is larger than the critical angle, the incident light is totally reflected. In other words, the light is trapped in the medium 801.
FIG. 16 shows the relation between angle of incident and reflectance when the medium 801 is glass (n1=1.52) and the medium 802 is air (n2=1.00).
As can be seen in FIG. 16, the reflectance sharply rises once the angle of incident to the interface reaches 35xc2x0 or larger. When the angle of incident to the interface is 41xc2x0 or larger, the light is totally reflected and cannot reach outside of the glass that is the medium 801.
A critical angle refers to the minimum angle at which total reflection of light at the interface between a medium 1 and a medium 2 takes place, and any angle larger than the critical angle causes total reflection. The magnitude of critical angle varies between media. For example, the critical angle is 41xc2x0 when the medium 801 is glass and the medium 802 is air whereas it is 42.2xc2x0 if the medium 801 is acrylic and the medium 802 is air.
Next, reference is made to FIG. 17. Reference numeral 202 denotes an organic compound layer. An arrow starting from the organic compound layer 202 shown in FIG. 17 indicates a direction in which light emitted from the organic compound layer 202 travels. The light emitted from the organic compound layer 202 is dispersed in every direction and enters the interface between the bottom face of a substrate 208 and air 209. Since light travels by nature toward a medium having higher index of refraction, only light that has a small angle of incident with respect to the interface between the substrate 208 and the air 209 can reach the air 209.
Assume here that the substrate 208 shown in FIG. 17 is a glass substrate (having an index of refraction of 1.52). Then, of the light emitted from the organic compound layer 202, one having an angle of incident of 35xc2x0 or larger and 41.1xc2x0 or smaller is reflected at the interface with abruptly increased reflectance. Furthermore, light having an angle of incident larger than 41.1xc2x0 is totally reflected at the interface and therefore cannot be taken out the substrate 208. Accordingly, the efficiency is low in taking out light emitted from the organic compound layer 202 to the exterior.
To simplify the explanation, this specification focuses on light refracted or reflected at the interface between the substrate 208 and the air 209 while ignoring other light emitted from the organic compound layer 202, namely, one that is refracted or reflected at the interface of solid thin films such as a gate insulating film and an interlayer insulating film. In actuality, light is always totally reflected or refracted at the interface between different media. For example, the interface between a transparent electrode and an interlayer insulating film, or the interface between an interlayer insulating film and a gate insulating film causes total reflection or refraction of light. However, these are ignored in this specification.
The present invention has been made to solve the above problem, and an object of the present invention is therefore to provide a light emitting device in which light emitted from a light emitting element can be taken out efficiently.
The structure of a light emitting device according to the present invention will be described with reference to FIG. 1A. In FIG. 1A, an arrow started from an organic compound layer 202 represents light emitted from the organic compound layer 202.
In FIGS. 1A and 1B, reference numeral 201 denotes a transparent electrode (pixel electrode); 202, the organic compound layer; and 203, a cathode. An area in which the transparent electrode 201, the organic compound layer 202, and the cathode 203 overlap one another corresponds to a light emitting element 200. 220 denotes a light reflector. Denoted by 208 is a substrate having an insulating surface; 209, air; 205, a first interlayer insulating film; and 204, a second interlayer insulating film. 207 denotes a passivation film.
In the present invention, the light reflector 220 is formed near the organic compound layer 202 as shown in FIG. 1A. The light reflector 220 is slanted with respect to light the organic compound layer 202 emits vertically to the substrate 208. In this specification, the angle of the light reflector 220 with respect to light the organic compound layer 202 emits vertically to the substrate 208 is called a taper angle of the light reflector 220.
A light reflector defined herein is an object that has a function of reflecting light. Specifically, the term refers to an object that has a function of reflecting light that has been emitted from an organic compound layer and transmitted through a solid thin film.
Next, reference is made to FIG. 1B. FIG. 1B is an enlarged view of an area of FIG. 1A that is surrounded by the dotted line. To simplify the illustration, the transparent electrode 201 is omitted in FIG. 1B. Light emitted from the organic compound layer 202 at a large exit angle xcex8a abuts against the light reflector 220 to be reflected. Then the light enters the interface between the substrate 208 and the air 209 at an angle of incident xcex8c. At this point, the angle of incident xcex8c is always smaller than the exit angle xcex8a because the light reflector 220 is slanted with respect to the substrate 208. As a result, light reflected at the light reflector 220 can reach the air 209.
The dotted line arrow of FIG. 1B shows the direction in which light travels when the light reflector 220 is not provided. Of light emitted from the organic compound layer 202, one that has a large exit angle xcex8a enters the interface between the substrate 208 and the air 209 at an angle of incident xcex8a. In this case, the angle of incident xcex8a is equal to the exit, angle xcex8a and is large. Therefore the light is totally reflected at the interface between the substrate 208 and the air 209 and cannot reach the air 209.
Of light emitted from the organic compound layer 202, one that has a small exit angle xcex8a does not travel toward the light reflector 220 and therefore can reach the air 209 (outside) without being influenced by the light reflector 220.
As described above, the present invention uses the light reflector 220 to have light, which cannot reach the air 209 in prior art, reflected so as to reach the air 209. To elaborate, light emitted from the organic compound layer 202, which otherwise would meet total reflection at the interface between the substrate 208 and the air 209, is reflected at the light reflector 220 to thereby reach the air 209. The efficiency in taking out light emitted from the organic compound layer 202 thus can be improved.
The description given next with reference to FIG. 13 is about the taper angle of the light reflector provided in the light emitting device of the present invention. FIG. 13 is an enlarged view of an area of FIG. 1B that is surrounded by the dotted line, and shows a case in which light emitted from the organic compound layer 202 is reflected at the light reflector 220 to be taken out of the substrate 208.
In FIG. 13, xcex8a represents the exit angle of light emitted from the organic compound layer 202, whereas xcex8b represents the taper angle of the light reflector 220. The taper angle is an angle of the light reflector 220 with respect to light the organic compound layer 202 emits vertically to the substrate 208. xcex8c represents the angle of incident at which the light reflected by the light reflector 220 enters the interface between the substrate 208 and the air 209. An angle obtained by subtracting xcex8b from xcex8a is given as xcex8e in the following Mathematical Expression 2.
Mathematical Expression 2
xcex8e=xcex8axe2x88x92xcex8bxe2x80x83xe2x80x83(2)
xcex8c is equal to an angle obtained by subtracting xcex8e from xcex8b as shown in the following Mathematical Expression 3.
Mathematical Expression 3
xcex8c=xcex8bxe2x88x92xcex8exe2x80x83xe2x80x83(3)
From the above Mathematical Expressions 2 and 3, Mathematical Expression 4 showing the relation among xcex8a, xcex8b, and xcex8c can be obtained.
Mathematical Expression 4
xcex8c=2xcex8bxe2x88x92xcex8axe2x80x83xe2x80x83(4)
When the substrate 208 is a glass substrate, the substrate 208 has an index of refraction of 1.52. Since the air 209 has an index of refraction of 1.0, the critical angle at the interface between the substrate 208 and the air 209 is 41xc2x0 from the graph of FIG. 16. This means that light with xcex8a larger than 41xc2x0 cannot be taken out of the substrate 208 unless the light reflector 220 is provided. Accordingly, only light with xcex8a equal to or larger than 41xc2x0 is taken into consideration. With regard to xcex8c, in order to take light reflected at the light reflector 220 out of the substrate 208, xcex8c has to satisfy the following Mathematical Expression 5.
Mathematical Expression 5
xe2x88x9241xc2x0 less than xcex8c less than 41xc2x0xe2x80x83xe2x80x83(5)
The following Mathematical Expression 6 can be obtained by substituting the Expression 4 for xcex8c in the Expression 5.
Mathematical Expression 6
xe2x88x9241xc2x0 less than 2xcex8bxe2x88x92xcex8a less than 41xc2x0xe2x80x83xe2x80x83(6)
In the Expression 6 is rearranged to obtain the following Mathematical Expression 7.
Mathematical Expression 7
(xe2x88x9241xc2x0+xcex8a)/2 less than xcex8b less than (41xc2x0+xcex8a)/2xe2x80x83xe2x80x83(7)
In the case considered here, xcex8a is 41xc2x0 or larger. Therefore, first, xcex8a in the Expression 7 is substituted by xcex8a=41xc2x0 to obtain the following Mathematical Expression 8.
Mathematical Expression 8
0xc2x0 less than xcex8b less than 41xc2x0xe2x80x83xe2x80x83(8)
Next, xcex8a in the Expression 7 is substituted by xcex8a=42xc2x0 to obtain the following Mathematical Expression 9.
Mathematical Expression 9
0.5xc2x0 less than xcex8b less than 41.5xc2x0xe2x80x83xe2x80x83(9)
Next, xcex8a in the Expression 7 is substituted by xcex8a=60xc2x0 to obtain the following Mathematical Expression 10.
Mathematical Expression 10
8.5xc2x0 less than xcex8b less than 50.5xc2x0xe2x80x83xe2x80x83(10)
Next, xcex8a in the Expression 7 is substituted by xcex8a=90xc2x0 to obtain the following Mathematical Expression 11.
Mathematical Expression 11
24.5xc2x0 less than xcex8b less than 65.5xc2x0xe2x80x83xe2x80x83(11)
As described above, the range of xcex8b varies depending on the value of xcex8a. From the Expression 11, xcex8b less than 65.5xc2x0 when xcex8a takes the maximum value (=90xc2x0). Accordingly, the maximum value of xcex8b is 65.5xc2x0. In other words, there is no need to set xcex8b to an angle larger than 65.5xc2x0.
When it is not necessary to set xcex8b to an angle larger than 65.5xc2x0, the substrate 208 is a glass substrate. Next, the critical angle at the interface between the substrate 208 and the air 209 is given as xcex8f and the maximum value for the taper angle xcex8b of the light reflector 220 is obtained. The following Mathematical Expression 12 is obtained by substituting the critical angle xcex8f for the critical angle in the Expression 7 above.
Mathematical Expression 12
(xe2x88x92xcex8f+xcex8a)/2 less than xcex8b less than (xcex8a+xcex8f)/2xe2x80x83xe2x80x83(12)
Here, the maximum value of xcex8a (=90xc2x0) substitutes xcex8a in the Expression 12 to obtain the maximum value of xcex8b.
Mathematical Expression 13
xcex8b less than (90xc2x0+xcex8f)/2xe2x80x83xe2x80x83(13)
By rearranging the Expression 13, the following Mathematical Expression 14 can be obtained.
Mathematical Expression 14
xcex8b less than 45xc2x0+(xcex8f/2)xe2x80x83xe2x80x83(14)
In conclusion, the taper angle of the light reflector 220 preferably set to satisfy the Expression 14.